The invention relates to a method of controlling the tension of an advancing yarn and which is adapted for use in a yarn false twist texturizing process.
DE 33 06 594 discloses a method of false twist texturing an advancing yarn, in which the twist torque imparted by the friction false twist unit to the yarn is adjusted as a function of the tension, in that the contact pressure of two surfaces acting upon the yarn is adjusted accordingly. This method allows the yarn tension to be adjusted to a constant value. The disadvantage of this method resides in the fact that fluctuations of the mean value are no longer obvious and, therefore, defects or errors that are to be detected by measuring the yarn tension can no longer be detected. For example, changes in the yarn tension may occur as a result of the wear of a feed system or errors in the temperature control of the texturing zone. However, these defects cannot be detected by the known method. Instead, these defects are corrected and thereby hidden.
EP 0 439 183 discloses a method of monitoring the tension of an advancing yarn in the texturing zone of a false twist crimping machine, wherein the yarn tension is corrected in that same is converted, via a time filter, into an adjustment signal, which controls the magnitude and/or the distribution of components of the frictional force exerted by the false twist unit on the yarn, the adjustment signal being used as a signal representing the continuous mean value of the continuously measured value for purposes of monitoring the quality. The adjustment signal which corrects the yarn tension is thus monitored to the effect, whether or not it leaves a predetermined range between an upper limit value and a lower limit value. These limit values are used to release an alarm signal, should the adjustment signal leave the range between these limit values. In addition, the difference between the actually measured yarn tension may be compared with the adjustment signal after a corresponding conversion, and an alarm signal may be released, should the difference signal leave a predetermined range between an upper and a lower limit value.
As described in WO 92/11535, a method of controlling the tension of an advancing yarn downstream of the friction false twist unit of a false twist crimping machine is based on the method described in EP 0 439 183, which relates to the adjustment for the control of the yarn tension in the twisting zone. A twist/advance ratio (D/Y) defined as quotient from the active radius of the friction false twist unit and the yarn speed is adjusted, in that the point of engagement of the yarn on the friction false twist unit and/or the yarn speed are adjusted.
Furthermore, EP 0 207 471 describes a method of monitoring the quality of an advancing yarn. This method serves primarily the purpose of detecting the defects that occur in the method described in DE 33 06 594.
In all the known methods or the prior art apparatus operating by these methods, it has been found that, while the numerous, individual friction false twist units of a false twist crimping machine are all of the same construction, the yarn tension surprisingly varies in the different positions, i.e. the different friction false twist units, or even over time. As a result, it is not possible to produce, even with a single false twist crimping machine, crimped yarns of consistent quality.
It is therefore the object of the present invention to provide a method of controlling the tension of an advancing yarn downstream of the friction false twist unit of a false twist crimping machine, which permits local and chronological variations in the crimp quality of the yarn to be minimized.